The PI seeks to develop a career as a physician-scientist focused on the problem of restoring and maintaining immune tolerance in individuals with Type 1 diabetes (T1D) by extending his scientific training in the exceptional environment at Vanderbilt University Medical Center. The failure of immune tolerance is an important underpinning of all human autoimmune diseases. When tolerance fails, autoimmune processes damage vital organs in tens of millions of patients worldwide. The applicant will focus on this scientific challenge through fundamental investigation of the most common pediatric autoimmune disorder-T1D, which afflicts more than 2 million Americans. To prevent and reverse this disease, it is necessary to restore immune tolerance to islet antigens. Most present efforts focus on either limiting the activation of islet-destructive T lymphocytes or enhancing the capacity of certain islet-protective T cells for immune regulation. Significantly, B lymphocytes also play a key role in the disease process both through the production of autoantibodies, which predict disease risk, and by the action of B lymphocytes as requisite antigen presenting cells. Targeting these cells with the B cell depleting agent rituximab has been at least as effective as T cell targeted agents, such as anti-CD3, in slowing the progression of new-onset diabetes. Moreover, studies in a relevant murine model of human T1D suggest a role of regulatory B cells in this salutary effect. These regulatory functions of B cells have also been observed in other models of autoimmune disease, and the applicant has previously characterized their role in transplantation tolerance. Building on these seminal data, we hypothesize that B lymphocytes play a regulatory role in the establishment and maintenance of peripheral immune tolerance and that this capacity is disrupted in T1D. This proposal investigates testable hypotheses which will focus on defining the regulatory functions of B lymphocytes that contribute to islet tolerance by determining: the role of classical B cell subsets, antigen specificity and developmental signals in regulatory B cell function (Aim 1), and their cellular targets and the mechanisms through which their regulatory function is controlled (Aim 2). The environment at Vanderbilt University is poised to address this fundamental biologic question given the local expertise in B lymphocyte immunobiology, the exceptional training atmosphere created by the Vanderbilt Diabetes Center, and the dedication to physician-scientist development exemplified by the Departments of Pediatrics and Microbiology and Immunology. Investigation of these novel hypotheses under the guidance of an exceptional mentoring committee and supported by these outstanding resources will allow the PI to realize a significant opportunity for new discovery in this clinically relevant field of inquiry-the fundamental problem of lost tolerance in T1D.